1. Field of the Invention
The field of the invention relates to an inhalation anaesthesia delivery systems generally, and more particularly to an inhalation anaesthesia delivery system comprising a fresh gas feeding arrangement connected to a breathing circuit, a monitor device, a control device and an interface unit, whereby the fresh gas feeding arrangement and the ventilator are configured to deliver a desired concentration of gas to the breathing circuit, and to a method of operating the same.
2. Description of Prior Art
The inhalation anaesthesia delivery systems are used to maintain oxygen (O2) and carbon dioxide (CO2) exchange of the patient during anaesthesia. During inspiration oxygen enriched gas flows into the lungs where it is diffused into circulation. At the same time CO2 is diffusing from the circulation into the lungs. During expiration the oxygen depleted and CO2 enriched gas flows out from the lungs. Inspiration and expiration together form a breath. The amount of delivered gas in single breath is called tidal volume. Inspiration may be either spontaneous action of the patient or artificial when a ventilator pressurizes the lungs with fresh gas. Expiration is spontaneous in both cases and effected by lung elastic forces.
Breathing gas is often mixed with anaesthesia gases to provide The inhalation anaesthesia. These gases are nitrous oxide (N2O) or air, and volatile anaesthetics. Patient concentration determines gas exchange status (O2, CO2) and the depth of anaesthesia of the patient. Normal range for N2O is 30%-75% and for volatile agents depending on the agent from 0.7% (halothane) to 3% (sevoflurane, enflurane, isoflurane) and up to 6%-12% with desflurane. CO2 concentration is typically about 5% and O2 concentration 25%-75%. Patient concentration is the best measured as end-expiration (=end-tidal) breathing gas concentration. However, oxygen delivery is often measured with inspiration gas O2 concentration.
Anaesthesia is delivered using rebreathing circuit where expired gas is circulated after removal of carbon dioxide and adding fresh gas back to inspiration. The proportion of circulated gas increases with decreasing fresh gas flow. To save anaesthetic gases, the fresh gas flow is minimized. In low flow, minimal flow, and closed circuit anaesthesia the circulated gas conforms the majority of the new breath. During anaesthesia, oxygen delivery is fitted with patient oxygen demand. In case delivery is low compared to demand patient oxygen reservoir is emptying and vice versa. Difficulty to control the oxygen reservoirs arises in low fresh gas flows where the system time constant for the control may be measured in tens of minutes. This can be shortened with temporary major increase of the fresh gas flow. Similar increase boosts also anaesthetic agent delivery change when required, but if this is not expected, vaporizer setting needs to be compensated for the fresh gas flow change. These back-and-forth controls made manually grab the attention of the anaesthesiologist from the patient. Recent development asks for increase in safety and efficacy, which promotes automatic control loops controlling the breathing actuators in response to the measured values. Increased automation free up the human resources in operating room to concentrate on patient instead of the machine, or even take care of more patients at the same time.
The filling grade of body oxygen reservoirs can be measured with end-expiration gas oxygen concentration (EtO2), arterial blood hemoglobin oxygen saturation (SpO2), or arterial blood oxygen partial pressure (PaO2). EtO2 and SpO2 are continuous non-invasive measurements whereas the PaO2 is discrete and minimally invasive. SpO2 is insensitive on changes when the blood hemoglobin oxygen reservoir is filled and gives a delayed response only when that reservoir is already emptying. EtO2 reflects the status of all body oxygenation reservoirs including the blood hemoglobin saturation and dissolved oxygen content in blood. However, EtO2 suffers of the problem that some lung ventilation/blood perfusion mismatch conditions may disturb the connection between the measured EtO2 and body oxygen storage status. The same may occur in case the circulatory status of the patient becomes disturbed e.g. due to reduced heart pumping action. As a result of such disturbances, EtO2 measurement may overestimate the filling grade of the body oxygen reservoirs.
Expired gas control principle is not a novelty regarding anaesthesia gas delivery. Preliminary research concepts to control volatile agent vaporizers based on measured end-tidal concentrations have been presented already on 1980'ies and is today commercially available. However, known O2 control systems match either inspired gas O2 concentration US 2005/0103338A1; US2006/0090757) or patient blood measured hemoglobin (Hb) oxygen saturation (SpO2) (US 2005/0109340; U.S. Pat. No. 6,761,165) with respective setting. Disadvantage of the inspired gas control is vague correlation with patient oxygenation state, specially during instable situations where the patient O2 level is changed. Actually, O2 inspired-to-expired difference remains even at steady state whereas the difference disappears with anaesthesia gases. Controlling SpO2 involves a problem of poor sensitivity when patient oxygenation is normal, i.e. SpO2 is above 95%. On the other side, below 90% body O2 reserves are already depleted and SpO2 changes rapidly in any changes in body oxygenation state. Time constant to affect the SpO2 by changing the O2 delivery is counted at minimum tens of seconds and responding this severe situation becomes thus delayed. The problems in SpO2 rely back on S-shaped Hb saturation dissociation curve describing the saturation on the ordinate and blood O2 partial pressure on abscissa. EtO2 is closely correlated with blood O2 partial pressure and is easily measurable at the point of care with any fast responding O2 sensor capable to separate inspiration and expiration concentrations.